1. Field of the Invention
This application relates generally to systems and methods for the purification of water. More particularly, it concerns effective removal of nitrogen compounds and suspended solids from wastewater treatment plant secondary effluents, especially when the nitrogen is predominantly in the nitrate or nitrite form.
2. Description of the Prior Art
With increasing concern over the quality of lakes, rivers and like national, state and municipal water bodies, it has become of critical importance to substantially reduce the level of nitrogen in wastewater effluents that are being discharged in a manner that is destructive to such water bodies. Limits on nitrogen content of such effluents have become necessary to prevent eutrophication and fish kills. Nitrogen is a fertilizer and promotes the growth of aquatic weeds, grasses, and algae. It can also deplete the dissolved oxygen levels in receiving waters, a situation that is toxic to aquatic life.
The concern over the discharge of nitrogen and research and development efforts to control this discharge began in the late 1960's. Discharge limits for nitrogen content in effluent water discharge generally range from 3 to 10 mg/l, with more recent requirements frequently being less than 3 mg/l.
Typically, after conventional primary and secondary treatment, including nitrification, up to about 30% of the nitrogen content has been eliminated and the remainder converted to nitrates and nitrites. These nitrate and nitrite compounds must be eliminated with further advanced wastewater treatment. There have been many processes used in the past for removing the remaining nitrogen compounds, including ion-exchange, suspended growth systems, fluidized bed systems, expanded bed systems, and biological denitrification. This latter process uses micro-organisms to reduce nitrate and nitrite nitrogen into nitrogen (N.sub.2), nitrous oxide (N.sub.2 O), or nitric oxide (NO).
In addition to nitrogen removal (NR), it has become necessary to reduce the total suspended solids (TSS) further, beyond secondary treatment, before discharging into convenient water bodies. Such final nitrogen and suspended solids reduction falls under the category of tertiary treatment.
Another type of denitrification system frequently used is a suspended growth reactor. It can be used as a separate denitrification system, following the nitrification stage, with methanol or other carbon source (see U.S. Pat. No. 3,709,364). Suspended growth reactors, with high recycle rates, can also be used for combined carbon oxidation-nitrification-denitrification systems, using wastewater instead of methanol as a carbon source. All suspended growth systems require clarifiers and recycle pumps for biomass return, so this increases their cost significantly. Recycle rates will range from 1-2:1 for a separate stage to 5-10:1 for the combined system.